A nucleic acid fragment having binding activity to a target molecule is referred to as a “nucleic acid aptamer,” and extensive applications thereof as nucleic acid pharmaceuticals to medical practice have been expected. A nucleic acid aptamer can be prepared via in vitro selection (the SELEX technique) by selecting and isolating a nucleic acid fragment that binds to a target molecule from a library of nucleic acid fragments comprising random nucleotide sequences.
vWF is a blood coagulation factor existing in the blood, and it is known that genetic mutation thereof is involved in the von Willebrand's disease and the like, and that acquired thrombotic thrombocytopenic purpura and the like are induced upon production of an autoantibody to vWF. In the past, several nucleic acid aptamers binding to vWF have been developed (Non-Patent Literature 1 and Non-Patent Literature 2). In comparison with an antibody, which is a protein composed of 20 amino acid species, a conventional nucleic acid aptamer is composed of 4 types of bases only, and variations thereof are limited. Accordingly, properties such as a binding ability, a dissociation rate, stability, and the like were not sufficient. Therefore, it was important to improve such properties, in order to use a nucleic acid aptamer in the medical field including treatment and diagnosis.